Masking is known to depend upon the relationship between the spatial-frequency content of target and mask. This relationship has been held constant in three experiments in order to investigate the separate contribution of the spatial parameters of the mask, in this case a grating with square-wave luminance profile. Thresholds for the detection of a probe target were highest when the background grating upon which the probe was superimposed had a spatial frequency of about 4 cycles deg-1 (experiment 1) and a duty cycle of 50% (experiment 3). In experiment 2, the thresholds were strongly affected by the size of the background grating even though the size of the target was small in proportion to the grating and remained constant. The increase in threshold was linearly related to the area of visual cortex to which the grating projected. The spatial parameters of gratings that maximise masking are therefore the same as those that have been shown to be optimal for the induction of perceptual distortions, suggesting a possible physiological mechanism for both the masking and the distortions.